In video processing, the luma (Y) and chroma (C) signal components are modulated together in order to generate a composite video signal. Integrating the luma and chroma video elements into a composite video stream facilitates video signal processing since only a single composite video stream is broadcasted. Once a composite signal is received, the luma and chroma signal components must be separated in order for the video signal to be processed and displayed.
A comb filter may be utilized for separating the chroma and luma video signal components. For example, a television set may be adapted to receive a composite video input, such as a composite video with burst and syncs (CVBS) input, for example, but the chroma and luma video components have to be separated before the television can display the received video signal.
FIG. 1A is a diagram illustrating the generation of a conventional composite video signal. Referring to FIG. 1A, adding the chroma signal component 102 and the luma signal component 104 produces a composite video signal 106. The luma signal component 104 may or may not increase in amplitude in a stair step fashion. The chroma signal component 102 may comprise a color difference component U modulated by, for example, a sine signal with a 3.58 MHz frequency and a color difference component V modulated by, for example, a cosine signal with a 3.58 MHz frequency. The modulated signals may be selected so that they provide quadrature modulation between the U and V color difference components. The modulation frequency corresponds to the frequency of a color sub-carrier signal. The luma signal component 104 increases in amplitude in a stair step fashion. The composite video signal 106 may be used in, for example, NTSC and PAL video standards.
FIG. 1B is a graphical diagram illustrating modulated chroma signals in contiguous composite video frames. The modulated chroma component is modulated at such a frequency that every line of video is phase-shifted by 180 degrees from the previous video line. Referring to FIG. 1B, the current video line in the “current frame” is phase-shifted by 180 degrees from the previous video line in the “current frame” as well as from the next video line in the “current frame.” Similarly, the current video line in the “previous frame” is phase-shifted by 180 degrees from the previous video line in the “previous frame” as well as from the next video line in the “previous frame.” In addition, since video field have a frequency rate of 59.94 Hz, there is a 180-degree phase shift between two adjacent frames, for example, the “current frame” and the “previous frame.” Correspondingly, the current video line in the “current frame” is 180 degrees phase-shifted from the current video line in the “previous frame.”
In conventional video processing, there are three ways to separate the luma and chroma video components—by utilizing a notch filter, by combing vertically or by combing temporally. During separation of the luma and chroma components, there are three bandwidth directions that may incur losses in the separation process and in the separated signal. Depending on the combing method that is utilized, the separated signal may have reduced vertical bandwidth, horizontal bandwidth, and/or temporal bandwidth.
The first way to separate the luma and chroma video components is by utilizing a notch filter. Since the components in a chroma signal are modulated at 3.58 MHz, a notch filter that is set at 3.58 MHz may be utilized. The notch filter, however, reduces the horizontal bandwidth in the output video signal. A comb filter delays a prior horizontally scanned line in order to compare it with a currently scanned line horizontal line. Combing vertically may also be utilized to separate the luma and chroma video components. Combing vertically may be achieved in three different ways—the current video line may be combed with the previous and the next video line, the current video line may be combed with the video line just before it, or the current video line may be combed with the video line just after it. The vertical combing is performed spatially, i.e., only within one field at a time and without any temporal combing. During combing in the “current frame,” for example, if the current video line is added to the previous video line, the chroma content cancels out and two times the luma content is obtained. On the other hand, if the previous video line is subtracted from the current video line, the luma content cancels out and two times the chroma content is obtained. In this way, luma and chroma content may be separated from the composite video signal for further processing. Comb filters provide better separation than notch filters because they suppress cross-color and cross-luma artifacts. In addition, vertical combing results in a reduced vertical bandwidth.
A third way to comb a composite signal is to comb temporally. Combing temporally comprises combing between two adjacent frames, for example, the “current frame” and the “previous frame”. Further, temporal combing is characterized by a reduced temporal bandwidth. The luma and chroma components may be separated by utilizing the same addition and subtraction methodology between a current video line and a previous video line, which is employed by vertical combing.
While 2-D comb filters are adapted to process successive scan lines for a single field of a video frame, 3-D comb filters are adapted to process scan lines that are taken from successive video frames. In general, for 3-D comb filtering, if there is motion between the successive video frames, a 3-D comb filter must revert to 2-D comb filtering. Motion includes color changes and image movement between frames. Accordingly, the 3-D comb filter is required to buffer at least one successive frame in order to determine whether there is motion between the buffered frames. In instances where there is color changes or image movements between the buffered frames, the corresponding Y/C components for the buffered frames will be different and the results of combing would be incorrect.
Before using comb filters to separate the luma and chroma video components, it may be necessary to process the video lines to ensure that the time length of each video line is the same. Changes in the timing of video lines produces errors in the luma and chroma separation because the corresponding addition and subtraction operations may not produce the necessary signal cancellation. While certain error in luma and chroma separation is generally accepted, timing variations may increase the error well above the level that the video system is capable of tolerating.
An ideal line locked system ensures that there are always a constant number of samples per scan video line regardless of whether the timing changes. During combing, conventional line locked systems may still produce artifacts. For example, some conventional line locked 2-D combing systems produce artifacts while combing multiple video lines in the same field. Similarly, some conventional line locked frame combing systems produce artifacts while temporally combing multiple video lines separated by one or more frames. Also, some conventional line locked 3-D combing systems produce artifacts while combing multiple video lines in the same field and multiple video lines separated by one or more frames. These artifacts generally result from variations occurring in the scanned video lines.
An ideal NTSC signal that conforms perfectly to the NTSC standard has a video line length of 63.55 μs. When this ideal NTSC signal is sampled at exactly 27 MHz, there are exactly 1716 samples per video line, for example, corresponding to approximately 37.04 ns of video line length for each sample. In a digital system, a constant delay, which may be an integer delay or integer plus a factional delay, may be constructed. The NTSC standard allows certain tolerable variations of the line length of up to about 0.001%. Notwithstanding, there are a number of devices which generate signals outside of the standard tolerable range for the video line length. VCRs, for example, may generate video line lengths that vary by ±5000 parts per million or ±317 ns. Assuming an ideal clock of frequency of 27 MHz, the variation in the video line length from an ideal line length of 63.55 us may require a system with a non-constant delay. Even if an incoming signal is always ideal, any variation in the frequency of the sampling clock may also require a non-constant delay. Variations in the frequency of the sampling clock may be due to factors such as temperature, voltage and/or printed circuit board (PCB) variations. Line locking circuits may be utilized to overcome these variations in the line length of the incoming signal and in the variations in the frequency of the sampling clock.
A line locked system utilizes a phase-locked loop (PLL) type mechanism, which measures the length of each incoming video line to sub-sample accuracy and adjusts the frequency of the sampling clock such that there are always 1716 samples per line. For example, if the line is shorter than 63.55 us, then the sampling frequency is increased. A line locking system creates a signal after sampling that is always a constant length. This allows the use of an easily implemented constant delay for video signal combing. However, line locked systems may need to maintain different clock domains in a single integrated circuit (IC) in order to provide the appropriate sampling frequency to the incoming signal and also provide the appropriate clock signal to the processing circuitry. Multiple clock domains are difficult to implement on an IC because of clock signal distribution requirements.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.